Neutral-colored gray photochromic plastic object

ABSTRACT

A neutral-colored gray photochromic plastic article which remains almost completely color-neutral or gray both during darkening as well as during bleaching. The plastic article according to the invention may be employed as a neutral-color gray photochromic ophthalmic lens.

BACKGROUND OF THE INVENTION

The invention concerns a neutral-color gray photochromic plastic articlewhich remains almost completely color-neutral or gray both whendarkening and when fading. The plastic article according to theinvention may in particular be used as a neutral-color gray photochromicophthalmic lens.

Photochromic plastic products, especially ophthalmic lenses have been onthe market since the Eighties. The first lenses to find a widerdistribution, e.g. the Rodenstock Perfalit Colormatic (since 1986) orthe tinted Transitions lens from Transitions Optical Inc. (since 1990)and offered by a number of lens manufacturers contained spiro-oxazinesas the photochromic dyes which darkened in blue hues. Upon moderatestimulation, this, combined with the brownish pre-coloration of thelens, produced an approximately gray color. Later products such as the“gray” Transitions Plus lens (since 1992), or the brown lensesTransitions Eurobraun and Hoya Sunbrown (since 1994), or the RodenstockGlas Perfalit Colormatic neu (since 1995) already contained pyrans aswell as spiro-oxazines and/or fulgides. Products currently availablecommercially such as the lens Transitions III preferably use pyrans,especially naphthopyrans and the larger ring systems derived from these.Of these, the Transitions III products with their refractive index of1.56 are based on U.S. Pat. No. 5,753,1461.

Various color systems have been developed and defined by which toobjectively characterize colors. The most widely known, and most suitedto ophthalmic lenses, is the L*a*b* or CIELAB color system (1976), inwhich the brightness (L*) and the color (a*, b*) can be represented by apoint within a sphere. During its darkening or fading process, aphotochromic lens may be described by a continuous set of points, i.e. acurve within a three-dimensional space. The principal factor inassessing the transmission color of an ophthalmic lens is the projectionof this curve onto a central plane, i.e. only the a* and b* values. Thesystem here is equispaced, i.e. the same color distances in the systemcorrespond to the same color differences.

In this color space, a lens which remains at the zero point with respectto a* and b* during the darkening and lightening cycle, i.e. which movesonly along the L* axis, represents the ideal case for a neutral-colorlens. It passes theoretically from white (=colorless since a lens,especially an ophthalmic lens, is considered in terms of transmission,not reflection) through all gray tones to black, i.e. to completeopacity. In all cases then, the lens is colorless or gray. Thismathematical target value is—as are the end points—not absolutelyattainable in reality, i.e. small deviations from the zero point in thea*b* plane are unavoidable.

U.S. Pat. No. 5,753,146 relates to compositions comprising at least twophotochromic naphthopyran compounds which are free of amino-substitutedaryl groups at the carbon atom adjacent to the oxygen atom of the pyranring. These compositions are intended, when inserted into an appropriatecarrier and exposed to ultraviolet solar radiation, to exhibit in theactivated state a gray or brown neutral color contained within the a*and b* ranges from +10 to −10 of the CIELAB color system. Measurementsof the lenses described as prior art in U.S. Pat. No. 5,753,146 haveshown that their color locus in the darkened state is at a* =+8 andb*^(˜)+4. During the fade phase, the values even reach a* =+12 andb^(˜)+14. For the chroma C* which is defined as the root of the sum ofthe squares of a* and b* and characterizes the deviation from the idealgray point, values are found in the darkened state measuring C*^(˜)9.0,and in the extreme case measuring up to C*^(˜)18.8. As a result, one canno longer speak in terms of a color-neutral lens; rather, the strongcolor shift during darkening and fade results in an effect which ischaracterized as the “chameleon effect.” Thus the lenses described inU.S. Pat. No. 5,753,146 as photochromic “gray” lenses, as well as otheravailable lenses of prior art, move a considerable distance duringdarkening and fade from the zero point. To be sure, based on measuresdescribed in U.S. Pat. No. 5,753,146, the color loci of lenses inquestion could be limited to smaller values. The C* value was halved to9.26; however almost the entire color curve is found in the green-bluecolor quadrant. The remaining high chroma value as well as theblue-dominated color curve are thus still far from the ideal of aneutral-color gray lens. For the known lenses of prior art, particularlyfor those described in U.S. Pat. No. 5,753,146, a pronounced color curveduring darkening and fade remain characteristic.

SUMMARY OF THE INVENTION

The object of the present invention is thus to produce a color-neutralgray photochromic plastic article which remains almost completely colorneutral, i.e. gray, both during darkening as well as fade.

This object is achieved by the embodiments characterized in the claims.Specifically, a color-neutral or gray photochromic plastic article isproduced which comprises incorporated into it at least two differentphotochromic color centers from the class of benzopyrans and higheranellated ring systems derived therefrom, the article beingcharacterized in that during a 15-minute exposure of 50 klux at 23° C.according to prEN 8980 and a 15-minute fade in the dark, it passes onlythrough color loci for which the chroma is C*<8, preferably C*<6, morepreferably C*<5.

In an especially preferred embodiment of the present invention, thecolor locus of the plastic article, after a 15-minute exposure of 50klux at 23° C. according to prEN 8980, exhibits a C* value of <5,preferably C*<4, more preferably C*<3.

In another embodiment of the present invention, the spectraltransmission of the article according to the invention is preferablyunder 25%, more preferably under 20% in the range of 400 to 560 nm inthe darkened state after a 15-minute exposure of 50 klux at 23° C.according to prEN 8980. The spectral transmission of the articleaccording to the invention is preferably under 50% in the range of 700nm in the darkened state after a 15-minute exposure of 50 klux at 23° C.according to prEN 8980.

An important aspect is the most constant transmission possible in thecentral range of the visible spectrum. Thus the spectral transmissiondifference in the range of 415 to 540 nm in the darkened state after a15-minute exposure of 50 klux at 23° C. according to prEN 8980 should,for example, be under 10%, more preferably under 8%.

In another embodiment, the spectral transmission according to V_(λ) inthe completely deactivated state given a 2 mm material thickness withoutantiglare measures is above 80%. Photochromic lenses are all-purposelenses; ideally they should replace both a clear lens as well as asun-protective lens. The resulting requirement is for the highesttransmission possible when not stimulated by light such as e.g. atnight. Combined with antiglare measures, the aforementioned value allowsfor transmission values approaching or at 90%.

The plastic article according to the invention may be employed as anoptical element such as a lens for which the refractive value measuredat the Na-d line lies between 1.49 and 1.76. In particular, the plasticarticle according to the invention may be employed as an ophthalmiclens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the absorption in the visible spectral range of a typicalexample according to the invention after a 15-minute exposure of 50 kluxat 23° C. according to the measuring procedure described in prEN 8980.

FIG. 2 show the color locus curve in the a*-b* color space of the CIELABsystem for a typical example according to the invention as compared withthe color locus curve of the example provided by U.S. Pat. No. 5,753,146(corresponding to the Transition gray lens 1.56) during a 15-minuteexposure of 50 klux at 23° C. according to the measuring proceduredescribed in prEN 8980 and a 15-minute fade in the dark at 23° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the present invention, for the first time a photochromicplastic article has been produced which remains almost completely colorneutral, i.e. in fact consistently gray, both during darkening as wellas during fade. The present invention is based on taking into accountspecific selection parameters with respect to photochromic dyes to beused in a photochromic plastic lens.

A neutral-color gray photochromic plastic article according to theinvention exhibits, for example, the spectral absorption shown in FIG. 1in the visible spectral range after a 15-minute exposure of 50 klux at23° C. according to the measuring procedure described in prEN 8980. Inthe range from 420 nm to 620 nm, the spectral transmission difference isunder 10%; this condition is maintained within a tolerance limit of ±10nm even during darkening and the first 15 minutes of fade. In the eventa lower absorption in the red spectral range is desired or acceptable,the neutral-color impression may, for example, be attained with a rangereduced to 600 nm.

The color locus of the photochromic plastic article which contains morethan one photochromic dye depends on the spectral composition of theexcitation light. This is true not only in the excitation phase (directsunlight or scattered light) but also in the fade phase for low lightlevels. In the framework of the present invention, the followingdiscussion utilizes a standard which is independent of all eventualitiesat the site of measurement such as the season or time of day, geographiclatitude, altitude, meteorological and climatic conditions, etc. The useof the solar simulator described in prEN 8980 permits an exposureapproaching natural conditions given varying temperatures and exposurelevels. An analysis of the transmitted light by a diode array andcomputer—as described in more detail in the technical section of prEN8980—allows for the determination of transmission and color locus every3 seconds. In the framework of the present invention, the color curvewas observed over a 15-minute period of darkening at 50 klux andsubsequently a 15-minute period of fade in the dark at a standardtemperature of 23° C.

According to the present invention, a color-neutral gray photochromicplastic article is one for which the color locus during the describeddarkening and fade continues to remain close to the zero point.Preferably all four quadrants of the CIELAB color system are passedthrough here. This may occur in either a clockwise or counterclockwisedirection, or in a loop movement (figure eight). Since the point oforigin is reached again usually only by special measures (e.g. baking)and at 23° C. not within 15 minutes of fade, the color locus curve isusually not closed. The ideal case of concentric circles surrounding azero point cannot be attained precisely; the shape of the color locusmotion is usually ellipsoid, square or rhombic. The important factor forthe neutral-gray color is the C* value. The color-neutral grayphotochromic plastic article according to the present invention exhibitsa C* value which at no time during the described cycle exceeds the valueof 8. Preferably, the color-neutral gray photochromic plastic articleaccording to the present invention exhibits a C8 value of C*<6, morepreferably C*<5.

Of special significance for the eyeglass wearer is the color locus inthe fully darkened state after 15 minutes exposure. Unlike the colorloci during darkening and fade, this is perceived in a quasi-stationarymanner, i.e. over an extended period of time. According to the presentinvention, the color-neutral gray photochromic plastic article exhibitsa color locus after a 15-minute exposure of 50 klux at 23° C. accordingto prEN 8980 with a C* value of <5, preferably with C*<4, especiallypreferably with C*<3.

It is generally known that a neutral hue is produced from additivemixing by mixing at least two colors. Given only two colors, theseshould be approximately complementary, i.e. the absorptions of thephotochromic dyes or their maxima must be connectable by a straight linepassing through the “white field” in the UCS chromaticity diagram of CIE1976 for 2° -standard observer. This means that given only two dyes, theabsorption maximum of the one dye should lie below about 496 nm, withthe second above about 570 nm. Values of approximately 100 to 130 nmthus result as the difference between absorption maxima for possiblecombinations.

The decisive factor for the true color locus of neutral-gray lenses is,however, not simply the position of the absorption maximum but the shapeof the absorption band in the visible spectral range. Unlike oxazines,pyrans exhibit a very broad “soft” structural maximum-wavelengthabsorption band in the visible spectral range. Additionally, differentpyrans, for example those described in WO 98/28289, possess twoabsorption bands in the visible spectral range, with theshorter-wavelength bands having nearly the same intensity as thelong-wavelength bands. These compounds are used preferably in caseswhere the second dye should still exhibit a pronounced absorption evenin the very long wavelength visible spectral range around 700 nm. Wheresuch broad absorption is desired, preferably two or more photochromiccompounds are used.

To prevent photochromic plastic lenses from displaying different colorsunder varying exposure conditions, e.g., early morning or late afternoonby contrast to the midday sun, the UV absorption is critical. UV heremeans not only the pure UV range up to 380 nm but also the very shortwavelength visible light up to about 400 nm. Here the decisive factor isnot so much the position of the UV maximum but rather the shape of theUV absorption bands. In terms of their absorption and concentration, thedyes must be selected such that no single dye takes up more than 60% ofthe intensity of the excitation light above 380 nm.

Where a dye exhibits several absorption bands of nearly equal intensityin the visible which safely cover the wavelength range from 420 to 580nm, then it is possible to obtain a neutral-color lens with only onephotochromic dye. This has the additional advantage that all factorswhich might impair a neutral color during darkening and fade, such asvarying darkening and fade rates, are eliminated. In this case, thecolor is also not affected by the temperature (no variable temperaturedependence) or the spectral composition of the excitation light (novarying excitation spectra).

In principle, photochromic plastic lenses may be produced in threedifferent ways. First, the photochromic dyes may be dispersedhomogeneously in the plastic lens material. Alternatively, they may beintroduced (usually only on the convex side) into the surface of theplastic lens by diffusion using heat. Additionally, they can be appliedas a layer on the plastic lens. For the first and last productionmethods, concentrations may be determined based on the individual dataof the specific photochromic dyes, while for diffusion-dyed lenses,other characteristic data are required. Critical here are, in additionto the dye used, the specific polymer matrix used, the diffusion rate,and the penetration depth therein of the dye. Of additional importanceare: the dyeing time, dyeing temperature, the type of intermediatecarrier (lacquer) used, and even the conditions of polymerization of theplastic lens.

The ideal solution for preparing a neutral-color gray photochromicplastic article would be, as described above, a single photochromic dyewith the appropriate absorption properties. In addition to theaforementioned advantages, all additionally cited problems and measuresrelated to balancing the dyes would be obviated. Currently, however,there is no known single photochromic dye with a single color centerwhich even approximates this property. Consequently, at least twophotochromic color centers are always required at the present time. Inthe framework of the present invention, however, the fact is taken intoaccount that the at least two color centers may be present in a singlemolecule in which these color centers are covalently linked, forexample, by spacer groups such as alkyl chains as described in DE 44 20378, for example.

According to the teaching of U.S. Pat. No. 5,753,146, a lens is dyedusing two pyrans that exhibit similar absorption maxima in the UV range,saturation absorptions, darkening and fade rates, and their maximumwavelength absorption maxima at 425 nm or 540 nm, is an especiallypreferred example. As was shown earlier, such a lens is never gray orbrown, however, but red to red violet. Completely ignored in U.S. Pat.No. 5,753,146 is the fact that especially pyrans with λ_(max)>525 nm areoften characterized by two or more absorption maxima in the visiblespectral range. These must be taken into account if their intensity isgreater than approximately 50% of the main band. Additionally, not onlythe position but also the shape and width of the absorption band(s) areof particular significance. In U.S. Pat. No. 5,753,146, the individualdyes and mixtures are introduced into the different plastics andaccording to differing dyeing methods. This makes little sense since thephotochromic dyes, especially the polar open form, exhibitsolvatochromic effects. Consequently, λ_(max) is dependent on thepolymer matrix used. This applies as well to light sensitivity andsaturation absorption, but most importantly to the fade rate.

In order therefore to produce the plastic article according to theinvention, the individual dyes to be employed must be tested in theplastic into which the mixture will later be incorporated. In the caseof incorporation by diffusion, the procedure is followed by which alldye parameters correlate to the process to be used later for production.First, in terms of the specific dyes to be employed, the full UV-VISspectra in the 300 to 780 nm range of absorption (extinction) arerecorded, both in the unexposed and exposed state after 15-minuteexposure at 23° C. From this, the spectral ΔOD curve may be generated bysubtraction (OD=optical density). The concentration of the dyes in thespecimens can be adjusted such that approximately equal ΔOD values maybe obtained in λ_(max) VIS.

To produce the plastic article according to the invention, a selectionmust then be made according to the spectral absorption behavior in theUV range. Here, as mentioned above, not just the λ_(max) UV but thetotal UV absorption is critical. First the absorption (extinction) inthe unexposed state is evaluated. Because some of the photochromic dyesin question absorb well beyond the UV limit, generally set at 380 nm, UVrange is understood to mean the wavelength range =410 nm in which thelight sensitivity of the average human eye is still less than about 0.1%of the maximum value. The absorption function is then combined with thespectral distribution of solar radiation E_(Sλ) (λ) when the use of thephotochromic plastic article under natural sunlight is to be theprincipal application. For this purpose, the values found in “proposedstandard solar-radiation curves for engineering use,” J. Franklin Inst.230 (1940) may be consulted. For other applications, the spectraldistribution of the radiation of the relevant excitation light source isused. For combined use, only those dyes are selected whose thusevaluated absorption integrals do not differ from one another by morethan a factor of 2. This is repeated in similar fashion for UVabsorption (extinction) in the exposed state.

To produce a plastic article according to the invention, the absorptionbehavior in the visible range is employed. For each dye, the absorptionrange can be determined in which the absorption ΔOD is at least 80% ofthe λ_(max) VIS value. To produce a plastic article according to theinvention, the dyes are selected so that the absorption ranges cover thespectral range from 420 to 600 nm such that the differences and/oroverlaps at the limit are ?10 nm. Where two or more dyes are used,mathematical absorption envelope curves must be computer-generated fordyes whose λ_(max) VIS difference is <50 nm. These may be treated likethe absorption curves of the individual dyes.

To produce a plastic article according to the invention, saturationabsorption does not have the prominent significance attributed to it inU.S. Pat. No. 5,753,146 since it may also be adjusted by modifying theconcentration of the dye in the mixture. In addition, it is a functionof the fade rate, i.e. photochromic dyes that fade more slowly alwayshave higher ΔOD values at saturation than do the faster-fading dyes ofrelated structure. Since inefficient photochromic dyes cannot beutilized economically, dyes available commercially differ in theirΔOD_(sat) only by a factor of 2 at maximum, as Table 1 of U.S. Pat. No.5,753,146 also shows. Since according to the Lambert-Beer law, theabsorption is an exponential function of concentration, this can beachieved with only small variations in concentration (at least as longas an approximately linear range is involved).

On this basis, the conditions for obtaining a neutral color impressionin the completely darkened states are met, this state being—as mentionedpreviously—of particular significance for the eyeglass wearer since itis experienced in a quasi-stationary manner, i.e. over an extendedperiod of time. With regard to the neutrality of the color locus duringthe entire darkening and fade process in terms of the production of aplastic article according to the invention, other factors must beconsidered such as the reaction rate of the dyes when darkening.Darkening over time is not a linear function. Under the citedmeasurement conditions at 23° C., currently available photochromicplastic ophthalmic lenses reach after 1 minute about 60% to 76% of themaximum darkening achieved after 15 minutes exposure (calculated inΔOD). The 50% value is reached in as little as 17 to 40 sec. In thisregard, an evaluation of the absorption after 5 sec used to estimate thevalue for one minute, as suggested in U.S. Pat. No. 5,753,146, producesan incorrect impression. In this amount of time, only 15% to 25% of thefinal value is reached. The “vertical-take-off” varieties of dye whichare overtaken in 10 sec by the rest and thus do not have a dominatingeffect are thereby overvalued. Even for this characteristic, absolutevalues have little meaning; rather, it is the reciprocal relation whichis important. To produce a plastic article according to the invention,the procedure can be followed by which the time is measured during whichthe photochromic dye in question reaches 50% of the final value. Thefaster (fastest) darkening dye should thereby not fall short of theslower (slowest) darkening dye by more than about 30%.

To produce a plastic article according to the invention, the matching offade times is of prominent importance. The fade or half-value period isthat period which the oil dye in the plastic article requires at 23° C.to retain half the absorption difference between the unexposed state andthe state after 15 minutes exposure. To maintain the neutral colorimpression over the 15-minute fade phase, the relative difference shouldnot exceed 25%. Contrary to the teaching of U.S. Pat. No. 5,753,146, theabsolute value or the absolute difference is meaningless for theneutrality of the color; rather it is the intended use of the plasticarticle which determines the parameters for the absolute value of thefade rate.

Since the individual skilled in the art does not have an unlimitedselection of photochromic dyes available to him/her, the ideal casedescribed in the introduction as prior art, i.e. an absolutely neutrallens for which the color locus always remains at the zero point withrespect to a* and b* during the darkening and fade cycle and moves onlyalong the L* axis, is only theoretically attainable; however, theprocedure described here allows for the production of a plastic articleaccording to the present invention which for the first time closelyapproaches this ideal case, i.e. the procedure actually yields aneutral-color gray plastic article or plastic lens.

It is possible to place the color locus in the completely darkened stateat the zero point of the a*b* coordinate system, but for the other colorloci, a deviation from this is unavoidable. However, such deviations arenot detrimental or are not perceived even by the color-consciouseyeglass wearer as color deviations if the chroma C* or chromadifference ΔC* is around 1 or less. This is understandable in that thecolor states are seen. sequentially, not side by side.

FIG. 2 shows the color locus curve in the a*b* color space of the CIELABsystem for a typical example according to the invention (mass-dyed inthe plastic material TS-150 from Tokuyama) and for Example 4 of U.S.Pat. No. 5,753,146 as a representative comparison sample (surface-dyedin CR-407 by PPG) during a 15 minute exposure of 50 klux according toprEN 8980 and a 15 minute fade in the dark at 23° C.

The chroma C* of the comparison example is 10.5 in the completelydarkened state, whereas the value for the example according to theinvention is C*<3. Additionally, at no time during darkening and fade isa C* value of 5 exceeded.

The following discussion explains in detail the procedure for producingthe neutral-color gray photochromic plastic article according to theinvention.

Reference is made to PCT-DE 98/02820 in regard to creating the specimensand to the measuring equipment.

As the carrier or matrix for the photochromic dyes to be employed orused, the plastic article according to the invention contains one ormore plastic materials. The plastic materials used can be those plasticsnormally used in prior art, especially for ophthalmic purposes. Forexample the plastic material may selected from among:poly(C₁-C₁₂-alkyl)methacrylates, polyoxyalkylene methacrylates,polyalkoxyphenol methacrylates, cellulose acetate, cellulose triacetate,cellulose acetate propionate, cellulose acetate butyrate, polyvinylacetate, polyvinyl alcohol, polyvinyl chloride, polyvinylidene chloride,polycarbonates, polyesters, polyurethanes, polyethylene terephthalate,polystyrene, poly-α-methylstyrene, polyvinyl butyrate, copoly(styrenemethylmethacrylate), copoly(styrene acrylonitrile) and polymers ofcomponents of the group, consisting of polyol(allyl carbonate) monomers,polyfunctional acrylate, methacrylate or diethylene glycoldimethacrylate monomers, ethoxylated bisphenol-A-dimethylacrylatemonomers, diisopropenyl benzene monomers, ethylene glycolbismethacrylate monomers, poly(ethylene glycol)bismethacrylate monomers,ethoxylated phenol methacrylate monomers, alkoxylated polyalcoholacrylates, and diallylidene pentaerythrite monomers or mixtures thereof.

First, all characteristic values are measured for the relevantphotochromic compounds in the material which is provided for theneutral-gray plastic article according to the invention. The specimensfor the characteristic values are produced in various ways depending onthe process by which the photochromic dyeing is to be performed. In thecase of mass dyeing, i.e. the admixture of the photochromic dyes beforepolymerization, the following method has proven to be effective. Thedyes are added to the prepared cast resin batch (monomers, initiators,any additives such as antioxidants, UV absorbers) in a concentration of0.0%1 to 0.1 wt. % , preferably 0.03% to 0.06 wt. %. Polymerization ofthese specimens is performed in the same polymerization program intendedfor the final plastic article. To determine the excitation maximum inthe UV or short-wavelength visible spectral range, the dyes must beincorporated in the same manner in a cast-resin batch without additives.In the case of photochromic dyeing of the plastic article by coating,the concentration is based on the thickness of the coating. For example,the quantities 2 wt. % for 20 μm and 1 wt. % for 40 μm have proven to besuitable. The product of the concentration of the dye in the completelycured layer in % of the coating weight and of the coating thicknessshould preferably be the value 4×10⁻⁷. If the process calls for thephotochromic surface dyeing of the plastic article by thermodiffusion,the concentration may be determined only with great effort, e.g. throughquantitative analysis or by UV absorption of the concentration series.The method has proven adequate, however, by which the dyeing conditions(temperature program) for a dye whose absorption maximum is in thevisible spectral range at about 550 nm are adjusted such that thespecimen exhibits an absorption difference of 0.7 before and afterstandard exposure—measured in ΔOD according to the V_(λ) of the spectrallight sensitivity of the human eye. All other samples are then producedusing exactly the same dyeing parameters with the other individualphotochromic dyes.

The specimens were measured, as described in the experimental section ofprEN 8980, on a kinetics test bench. The measured light passing throughthe specimen should be capable of being analyzed in rapid sequenceaccording to transmission and color locus, e.g. using a diode arrayspectrometer. The result of this measurement is the color locus of thespecimen before exposure and at any given time during the exposure andthe fade phase. The latter generally occurs in the dark. Depending theintended application of the plastic article, this may also occur understandard illuminant A (incandescent light) or under attenuated exposure(shadow state). At minimum, a spectral analysis is performed beforeexposure, at the end of exposure and at the end of the measurementperiod, i.e. an absorption spectrum is recorded in the range of about350 to 800 nm. The conditions for simulating natural exposure bysunlight, such as intensity and spectral distribution of the excitationlight, are established in prEN 8980; for ophthalmic lenses, ameasurement temperature of 23° C. and an exposure intensity of 50 kluxhave proven appropriate. However, other temperatures and exposures arepossible, depending on the later principal application of the plasticarticle. When the photochromic article is intended for use in a broadertemperature range, the temperature dependency of the photochromic effectis usually of significance. In this case, the samples are measured attwo temperatures which include at least 80% of the later control range.With ophthalmic lenses intended for the European markets, for example,these values are 10° C. and 35° C. Where the fatigue from aging of thephotochromic reaction is of significance, the samples are measured againafter artificial aging (e.g. 50 h in the Suntest device made byHeraeus).

The following data are then obtained from the spectral measurements:from the spectrum in the unexposed state, the longest wavelengthexcitation maximum in the UV (UV_(max)) and from the last spectrum atthe end of exposure, generally the completely activated state, thelongest wavelength absorption maximum in the visible spectral range(VIS_(max)). The measured difference for (VIS_(max)) in optical density(ΔOD) between the fully activated state and the unexposed state isdesignated L_(max). The difference from the unexposed state after 15 secexposure is designated L_(e). The ratio of L_(e) to L_(max) is V_(e).During fade, the time Z_(a) which the sample requires to fade inVIS_(max) from the fully darkened state in optical density by the valueL_(max)/2, is the critical parameter. The value Z_(a) is given inseconds.

If the temperature dependency is also required, the absolute differenceof the L_(max) values for the two limiting values, divided by the meanof the two L_(max) values, provides the required T_(a). This valueincreases according to the temperature dependency of the specimen. Theratio of the L_(max) values for the samples after and before aging isdesignated the aging value A.

The photochromic dyes are then listed in descending order in the firstcolumn of a table by their VIS^(max) value. The corresponding Z_(a)values are entered in the second A column, while the V_(e) values areentered in the third column. The UV_(max) values are listed in a fourthcolumn. The temperature dependency T_(a) may be entered in a fifthcolumn, while the aging value A may be shown in the sixth column.

For the neutral-gray article according to the invention, thephotochromic dyes used as the base dyes are those for which theVIS_(max) value is above 550 nm, i.e. one or more essentially bluenaphthopyran dyes, as explained below. Since the spectral lightsensitivity of the human eye is highest at this wavelength and itsproduct using the standard illuminant D 65 at 620 nm essentiallycorresponds to that at 500 nm, the subjective impression of thephotochromic reaction, i.e. the intensity of darkening, rate ofdarkening and fade, etc., is characterized principally by these dyes.These and the remaining dyes are therefore divided into two groups. Theselection of one (or more) photochromic dyes from this group oflong-wavelength dyes forms the basis, as shown below, for theneutral-gray plastic article according to the invention. The selectionis made depending on the specific requirements regarding the reactionrate for darkening and fade, temperature dependency as well asresistance to aging. VIS_(max) values around 590 nm have proven to beespecially preferred. If only one photochromic dye from the group above550 nm is selected, the VIS_(max) values above 620 nm and below 570 nmhave proven to be disadvantageous. In both cases, color correction ishampered; in the second case, the transmission in the red spectral rangeis so high that during extreme darkening, e.g. when skiing, distortionsof color may occur. Where two or more dyes are selected from this group,the means of their characteristic values (Z_(a), V_(e), T_(a), A) may betreated as a single dye so long as their relative values do not deviatefrom one another by more than 15%. Otherwise, the additional dyes to beadded for each correspondingly selected base dye must be selected asexplained below.

For a neutral-gray plastic article according to the invention, variouscriteria are—deviating from the information found in U.S. Pat. No.5,753,146—not valid, while others are valid only to a limited extent.Most importantly, it is not the absolute, but rather the relativedeviations of the dyes from each other which is of significance;L_(max), the maximum darkening performance of the dyes is of nosignificance. For the examples selected in U.S. Pat. No. 5,753,146,ΔL_(max) is 0.01. According to the invention, those with ΔL_(max)>0.2(or rel. 30%) may also be selected. Thus all other photochromic dyes areeliminated from the selection list for which the V_(e) and Z_(a) valuesdeviate by more than 30% from those of the reference dye, i.e. the basedye. A deviation by rel. up to 50%, as in the examples from U.S. Pat.No. 5,753,146, precludes the maintenance of the gray color duringdarkening and fade. On the other hand, the absolute value has nosignificance in terms of maintaining the neutral-gray color.

Where temperature dependency is important, of the remaining availablephotochromic dyes, all those are eliminated for which the T_(a) valuedeviates by more than 30% from that of the reference dye. Whereresistance to aging is important, of those remaining, all are eliminatedfor which the A value deviates by more than 20% from that of thereference dye. For the use of the photochromic article according to theinvention under highly varying exposure conditions, especially thosewith highly varying spectral intensities in the UV and short-wavelengthrange, as for example for ophthalmic lenses, the UV_(max) value is alsoof decisive importance. The relative intensity of short-wavelengthradiation at 360 nm vis-à-vis 390 nm (I₃₆₀/I₃₉₀) is, for example, muchlower in big-city smog than in high mountainous areas. If theneutral-gray color according to the invention is to be maintained notonly under the standard illuminant but also under these contrastingconditions, the UV_(max) values must not deviate by more than 15 nm fromthat of the reference dye.

Starting from the 100, for example, photochromic dyes from the group ofbenzopyrans and of higher anellated ring systems derived therefrom, suchas the naphthopyran dyes which are readily available to the individualskilled in the art, and based on the above criteria—as a function of theselected reference dye or base dye—the number of photochromic dyes to beadded for the production of the neutral-gray plastic article accordingto the invention is generally reduced to less than a dozen. In order tofurther reduce the additional photochromic dyes required to obtain aphotochromic plastic article according to the invention, the absorptionspectrum of the reference dye in the fully activated state is taken intoaccount. Viewed physically, an ideal gray lens has an absorption whichis constant over the entire visible spectral range from 380 nm to over780 nm. However, based on the spectral light sensitivity of the humaneye, this is not necessary from a physiological point of view. Theranges below 430 nm and chiefly above 680 nm are of secondarysignificance. First, those wavelengths are determined in which arelative absorption minimum is present. Then those dyes with thecorresponding VIS_(max), values are selected from the aforementionedtable which are suitable for closing this absorption gap. If, based onthe prior limitations, the selection is too small from which to find aprecisely matching dye, an appropriate dye mixture is used to fill thisabsorption gap. The total concentration to be used lies in the range of25% to 40 wt. % of the reference dye. For most applications, e.g. foruse as sun-protective lenses in eyeglasses, the concentration of thereference dye required for sufficient darkening lies outside the scopeof the Lambert-Beer law.

The compounds usable for the present invention all belong to the classof benzopyrans and higher anellated ring systems derived therefrom suchas the naphthopyrans and fluorenopyrans. For the upper wavelength range,these are chiefly: the 2,2 position aromatically or heteroaromaticallysubstituted [2H]-naphtho(1,2-b)-pyrans, while for the lower wavelengthrange: the [3H]-naphtho(2,1-b) pyrans correspondingly substituted in the3,3 position. Examples of long-wavelength absorbing dyes which mayfunction as reference or base dyes (λ_(max)=550 nm) include thenaphthopyrans described in PCT-DE 98/02820 and[2,1-f]naphtho[1,2-b]pyran derivatives and/or spiro-9-fluoreno[1,2-b]pyran derivatives described in PCT/EP 99/05258. A list ofpreferred examples hereof includes:

3,13-diphenyl-3-(4-diphenylaminophenyl)′-13-hydroxy-6-methoxy-indeno[2,1-f]-naphtho[1,2-b]pyran,

13-(2,5-dimethylphenyl)-3-(4-diphenylaminophenyl)-13-hydroxy-6-methoxy-3-phenyl-indeno[2,1-f]-naphtho[1,2-b]pyran,

13-(2,5-dimethylphenyl)-3-(4-diphenylaminophenyl)-13-hydroxy-3-phenyl-indeno[2,1-f]-naphtho[1,2-b]pyran,

spiro-9-fluoreno-13′-[3-(4-dimethylaminophenyl)-6-methoxy-3-phenyl-indeno[2,1f]naphtho[1,2-b]pyran],

spiro-9-fluoreno-13′-[3-(4-dimethylaminophenyl)-3-phenyl-indeno[2,1-f]naphtho-[1,2-b]pyran],

spiro-9-fluoreno-13′-[3-(4-dimethylaminophenyl)-6-methoxy-3-phenyl-indeno[2,1-f]naphtho[1,2-]pyran],

spiro-9-fluoreno-13′-[3-(4-diphenylaminophenyl)-3-phenyl-indeno[2,1-f]naphtho[1,2b]pyran],

spiro-9-fluoreno-13′-{3-[4-(N-morpholinyl)phenyl]-6-methoxy-3-phenyl-indeno[2,1f]naphtho[1,2-]pyran},

spiro-9-fluoreno-13′-{3-[4-(N-morpholinyl)phenyl]-3-phenyl-indeno[2,1-f]naphtho-[1,2b]pyran},

spiro-9-fluoreno-13′-{6-methoxy-3-phenyl-3-[4-(N-piperidinyl)phenyl]-indeno[2,1-f]naptho[1,2-b]pyran}and

spiro-9-fluoreno-13′-{3-phenyl-3-[4-(N-piperidinyl)phenyl]-indeno[2,1-f]naphtho-[1,2-f]naphtho-[1,2-b]pyran}.

Examples of shorter-wavelength absorbing photochromic dyes which may becombined with the base dye include:

3-(4-diphenylaminophenyl)-3-(2-fluorophenyl)-3H-naphtho[2,1-b]pyran,

3-(4-dimethylaminophenyl)-3-(2-fluorophenyl)-3H-naphtho[2,1-b]pyran,

3-(2-fluorophenyl)-3-[4-(N-morpholinyl)phenyl]-3H-naphtho[2,1-b]pyran,

3-(2-fluorophenyl)-3-[4-(N-piperidinyl)phenyl]-3H-naphtho[2,1-b]pyran,

3-(4-dimethylaminophenyl)-6-(N-morpholinyl)-3-phenyl-3H-naphtho[2,1-b]pyran,

6-(N-morpholinyl)-3-[4-(N-morpholinyl)phenyl]-3-phenyl-3H-naphtho[2,1-b]pyran,

6-(N-morpholinyl)-3-phenyl-3-[4-(N-piperidinyl)phenyl]-3H-naphtho[2,1-b]pyran,

6-(N-morpholinyl)-3-phenyl-3-[4-(N-pyrrolidinyl)phenyl]-3H-naphtho[2,1-b]pyran,

3-phenyl-3-(2-fluorophenyl)-3H-naphtho[2′,1-b]pyran,

6-(N-morpholinyl)-3,3-diphenyl-3H-naphtho[2,1-b]pyran and

6-(N-morpholinyl)-3-(4-methoxyphenyl)-3-phenyl-3H-naphtho[2,1-b]pyran.

However, the pyrans described in U.S. Pat. No. 5,753,146and EP-A-0 562915may also be used. In the same way, the average individual skilled inthe art is also familiar with the method of adding or admixing smallquantities of photochromic dyes of other classes, for example, oxazinesor fulgides, as e.g. described in U.S. Pat. No. 5,753,146, withoutdeparting from the gist of the present invention. Other usable oxazines(λ_(max)=600nm) are marketed by the HCH James Robinson Company. Forpurposes not entailing a long-term continuous load or for which this isplays only a secondary role, small quantities of fulgides may beadmixed, such as those offered by the Tokuyama Company although theirλ_(max) value should exceed 570nm.

Whereas the information or measures described in the prior art relatingto the preparation of a gray plastic lens are deficient or evenmisleading, the above-described method results in an article accordingto the invention, an actually neutral-color gray photochromic plasticarticle which remains almost completely color-neutral or gray evenduring fade. Any minor corrections are well within the technicalcompetence of the average individual skilled in the art.

What is claimed is:
 1. A neutral-color photochromic plastic articlecomprising a plastic host material and at least two differentphotochromic color centers selected from the group consisting ofbenzopyrans and higher anellated ring systems derived from benzopyrans,wherein said article passes only through those color loci whose chromais C*<8 during a 15 minute exposure of 50 klux at 23° C. according toprEN 8980 and a 15 minute fade period in the dark.
 2. A plastic articleaccording to claim 1, wherein the chroma of the color loci is C*<5.
 3. Aplastic article according to claim 1, wherein said article has a colorlocus which exhibits a C* value <5 after a 15-minute exposure of 50 kluxat 23 ° C. according to prEN
 8980. 4. A plastic article according toclaim 3, wherein the color locus exhibits a C* value <3.
 5. A plasticarticle according to claim 1, wherein said article has a spectraltransmission of less than 25 % in the 400 to 650 nm wavelength range inthe darkened state after a 15 minute exposure of 50 klux at 23 ° C.according to prEN
 8980. 6. A plastic article according to claim 1,wherein said article has a spectral transmission of less than 50 % at awavelength of 700 nm in the darkened state after a 15 minute exposure of50 klux at 23 ° C. according to prEN
 8980. 7. A plastic articleaccording to claim 1, wherein said article has a spectral transmissionof more than 80 % according to V_(λ)in the deactivated state for a 2 mmmaterial thickness without anti-reflection measures.
 8. A plasticarticle according to claim 1, wherein said article is an ophthalmiclens.